Adjust embedded languages in .rst files.

- Use syntax/rst.vim's region names so highlighting is consistent
  for both highlighted and unhighlighted language in code-blocks.

- Refactor function to embed languages to allow non-redundant
  implementation for code-blocks with unhighlighted languages.

- Warn about use of g:rst_syntax_code_list to avoid user surprise.
user/jszakmeister: cope with 8-color terminals better